Studies on the regulation of cell proliferation are important to our understanding of developmental biology and tumorigenesis. The long-term goal of the proposed research is to learn the molecular mechanisms that negatively regulate cell proliferation. In order to address this question, we are identifying growth-constraining tumor suppressor genes and studying their functions in the developing imaginal tissues of Drosophila melanogaster. Several aspects of this system make it uniquely suited for analyzing mechanisms that negatively regulate cell proliferation: (1) mechanisms are known to exist in controlling cell proliferation in imaginal tissues; (2) a newly developed genetic technique provides an unprecedented opportunity to identify mutations affecting such mechanisms in mosaic animals, and components of such mechanisms have already been identified; (3) additional components can be identified by future genetic screens; and (4) standard molecular and genetic techniques can be used to characterize these components. The three key genes to be studied are lats, "93B" and "2-1", in which recessive mutations can cause dramatic overproliferation of mutant cells in mosaic animals. The lats gene encodes a novel protein kinase homolog with a putative SH3-binding site. The specific aims of this proposal are: (1) to continue the molecular and genetic characterization of the lats, "93B", and "2-1" genes; and (2) to identify genetically and characterize additional genes involved in the negative regulation of cell proliferation or that interact with lats. The genetic screens will identify a network of genes involved in controlling cell proliferation during development. The genetic and molecular characterizations proposed will define the relationships among these genes and begin to assign biochemical functions which will aid in understanding their interactions at the molecular level. In this manner, we hope to learn how cell proliferation is regulated during normal development and how mutations could lead to abnormal growth and tumorigenesis.